Interestingly that's not what mr Dyno said. My car is a 2.5 though. Everyone talks about spool but it's a complex subject until you've had a few different turbos on the same car. The stock turbo is on boost quickly but even when it is it's a bit yawn..... The TD04 with the 'big' housing is just as quick onto boost but there's much more - to the point where it's quite hard to drive smoothly. You want a reasonable amount of acceleration but you move your foot a milimeter too far and your head is bashed into the headrest. Fun for a while but eventually annoying and possibly dangerous to transmission/engine longevity. The power drops off way before the redline. The TD05(6) is no slower onto boost but doesn't produce an annoying spike - the power builds in a smoother way. The effect in a drag race would be that the 4 car might snatch a car length in the first second but after that would be a dot in the mirrors of the 5 car. The stock turbo car would look like it's in reverse.....No way a TD05 or 6 will spool faster on an EU 2.0 than a TD04, launches with too large a turbo will be a PITA, you'll just about get on boost on a launch with a TD04, so would suggest that route myself, 260bhp is easy with a TD04, 280 a stretch (you drop about 20bhp to Impreza numbers due to the softer cams).
Simon
As has been said before, TD04 is the way to go.TD04 or TD05-That is the question: There aint much difference in the price of second hand ones. So which is it be on a UK 99 s turbo auto?